1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, printer and the like using electrophotographic methods.
2. Description of the Related Art
In image forming apparatus such as copiers, printers and the like using electrophotographic methods, typically the surface of a photosensitive drum is charged via a charger, and exposed to an optical image in said charged region so as to form an electrostatic latent image thereon which is then developed so as to be rendered visible, and transferred onto a transfer member upon which the developed image is fixed.
In recent years, various apparatus have been proposed which omit a cleaning device in accordance with demand for more inexpensive and more compact apparatus.
For example, U.S. Pat. No. 5,148,210 discloses a so-called cleanerless image forming apparatus which combines a cleaning device with a developing device.
Various types of charging devices are known, but such devices can be broadly divided into corona chargers which use a corona discharge, and contact chargers wherein a charging member makes contact with a latent image-bearing member. Contact chargers use a stationary brush, rotating brush, charging roller-type member, or rotatably driven belt-like charging member.
Charging devices using a corona discharge are advantageous in that provide stable charges, but are disadvantageous insofar as they generate large amounts of ozone which causes deterioration of the photosensitive member typically used as an image-bearing member, and which is extremely toxic to humans. Thus, contact chargers which produce markedly less ozone than corona charger are preferable.
Among contact chargers are brush-type chargers which, when used in the previously mentioned cleanerless type image forming apparatus, provide a brush in said charger that disrupts the residual developer remaining on the surface of the latent image-bearing member after image transfer, e.g., disruption via a rotating brush, and suppress inadequate charging of residual developer and inadequate optical exposure during subsequent image forming processes, and thereby suppress so-called memory generation (refer to U.S. Pat. No. 5,148,219).
In general, rotating brushes provide better charge stability than stationary brushes.
On the other hand, in the case of image forming apparatus having a cleanerless type construction wherein a cleaning device is combined with a developing device and which use a rotating charging brush as a charger, the residual developer remaining after image transfer cannot be adequately dispersed by just using a rotating brush in the charger. In general, when a rotating brush is used in a charger, residual developer remaining after image transfer is effectively dispersed when the transfer efficiency is about 85% or greater, but when the transfer efficiency is lower, the dispersion effectiveness is reduced.
When the transfer efficiency is 60%, for example, dispersion effectiveness is reduced and poor image quality readily results. When the residual developer is not adequately dispersed in the charging area, an unexposable region is generated on the surface of the latent image-bearing member which is to be originally exposed in the image exposure process, thereby preventing the satisfactory image formation.
In order to improve dispersion effectiveness, it was considered to rotate the rotating brush in a direction counter to the direction of movement of the surface of the latent image-bearing member, i.e., the rotating brush is rotated in a direction opposite to the direction of movement of said latent image-bearing surface, but in practice such an arrangement caused the developer to be strongly pressed against the surface of the latent image-bearing member so as to scratch said surface. Thus, the dispersion effectiveness of the developer after image transfer was lost, and so-called developer filming results which prevents image formation when such filming becomes pronounced.